(1) Field of the Invention
The invention relates to protection of displays from potentially damaging events. More specifically, the invention relates to protection of displays from DC voltage damage caused by fault conditions.
(2) Background
Liquid crystal displays have been known generally for many years. Initially, liquid crystal displays were formed on amorphous silicon substrates. Amorphous silicon tended to be slow, relatively large, and unsuited for formation of high speed logic and other device types. Liquid crystal displays then evolved into a polysilicon style which still had inadequate speed and logic compatibility characteristics. More recently, crystalline silicon has been employed to manufacture very small liquid crystal on silicon (LCOS) displays. These displays are much faster than the displays which use polysilicon to form display devices and can permit high speed driving of the individual pixels on display. In miniature displays, the challenge is to make the displays as small as possible and to also minimize power consumption, as these displays are increasingly used in a mobile environment with limited power resources.
The use of lookup tables (LUTs) is generally understood in the art. A LUT is effectively a listing of output codes which correspond to the possible input codes. The LUT performs a mapping of the input code to the output code, though the input code and output code may have different ranges. Typically, when LUTs are used in the context of displaying graphical data, three LUTs are employed to carry output codes corresponding to each of the color components (e.g. red, green, and blue) of a pixel. The subsequent three output codes are all simultaneously driven to the display to create the image of the color specified. Using three LUTs per pixel implies non-trivial die space to form the three LUTs, as well as power consumption by all those tables. In the context of miniature displays, it is desirable to minimize both die space and power consumption.
Because driving consecutive positive frames may cause liquid crystal malfunctions, liquid crystal display pixels are invariably driven by an alternating voltage having a positive and negative swing. While this places certain powering constraints on the system, it also necessitates use of frame inversion techniques in which the frame data is inverted to be driven by a negative signal. The required data inversion increases complexity of the display controller as well as power consumption and die are required.
It is also known in the art that liquid crystal displays can be damaged if exposed to DC voltage for a significant period of time. Thus, efforts have been made to reduce the risk of such damage. However, in the case of systems where the display is remote from the display controller, improper cable connections, a crash at the controller, or even a damaged cable can cause large and damaging voltages to be driven to a liquid crystal display. The problem arises how to protect the display from damages as a result of likely faults. Solutions to this problem are largely absent in the existing art.
An apparatus and method for protecting a display from fault conditions is disclosed. An oscillator independent of a display clock of a display to be protected is formed on a substrate with the display. The oscillator generates a periodic signal which is used to define a period during which a register collects indicators of normal system operation. Combinational logic is used to determine if the set of indicators collected matches the set of indicators expected during the time period. If the set matches, operation is deemed normal and no action is taken. If an indicator is missing, the combinational logic generates a signal that causes the display to be driven into a safe state until such time as all indicators are again collected within the defined time period.